1. Field of the Invention
The present invention relates to a data communication system and more particularly, relates to a data communication system adapted such that voice communication as well as data communication is made possible.
2. Description of the Prior Art
Such a data communication system has been used in an emergency telephone or informer system in which an emergency call is possible, for example. FIG. 1 is a schematic block diagram showing a conventional emergency informer system used in an elevator, in which system an emergency call is possible. A receiving center 1 for supervising a failure of an elevator is connected through a telephone line 3 to an emergency call unit 2 having an emergency call capability, provided with the elevator. At the time of emergency, the emergency call unit 2 is connected to a receiving center 1 through the line 3 by depressing a call request switch 21. A circuit responsive to depression of the call request switch 21 for providing a line between the emergency call unit 2 and the center 1 is not shown. Then, a predetermined emergency call is made from the emergency call unit 2 to the center 1 through an operation of modems 5A, 5B and communication controls 6 and 14. More particularly, first a predetermined emergency call code is produced in the communication control 14 included in the emergency call unit 2. Next the code is modulated by the modem 5B and is sent to the center 1 through the line 3 in the center 1, and the sent signal is demodulated by the modem 5A and then is applied to the communication control 6 in the center 1. The communication control 6 makes a predetermined emergency call such as elevator emergency stop, when the control 6 detects the emergency call code. In addition, an operator on the center side and a passenger can speak to each other using a handset 4 in the center 1 and a microphone 19 and a passenger 20 in the elevator, respectively. However, if both of them are adapted such that simultaneous transmission/reception becomes possible such as a subscriber's station, voice from the speaker 20 is applied to the microphone 19 and hence a hauling phenomenon occurs. In order to prevent this phenomenon, a remote press-to-talk method has been used wherein an operator on the center 1 side actuates a press-to-talk switch 10 so that an L relay 16 in the emergency call unit 2 is controlled to be switched. The operation is made by detecting a voice communication control code sent from the center 1 by a voice communication control code detecting portion 15 in the emergency call unit 2 and by controlling the L relay 16 in accordance with the content of the detected code to switch a transmission/reception by a contact 17.
Now the remote press-to-talk operation will be described in detail. An operator in the center 1 turns a call request switch 9 off in response to an emergency call from the emergency call unit 2. As a result, an M relay 7 is released and the handset 4 is connected to the line 3, so that the operator can listen to voice of a speaker on the side of the emergency call unit 2 through the handset 4. If and when the operator wishes to send his voice to the speaker 20, he depresses the press-to-talk switch 10. Then, a monostable multivibrator 11 is triggered and hence a logical "H" pulse having a predetermined pulse width appears in the output Q thereof. Since the pulse renders a transistor 12 conductive, the M relay 7 makes a timing operation corresponding to the pulse width. Correspondingly, an m contact 8 operates and the line 3 is connected to the modem 5A side. On the other hand, the output Q of the monostable multivibrator 11 and the state of the press-to-talk switch 10 are inputted to the communication control 6. If and when an input A to the control 6 is a logical "L" (a press-to-talk switch 10 is on) and an input B is a logical "H", the communication control 6 sends out to the modem 5A a receiving code which is a voice communication control code for switching the emergency call unit 2 to the receiving side. The receiving code is modulated by the modem 5A, passes through the actuating m contact 8, and is detected by a voice communication control code detecting portion 15 through the line 3, a hybrid coil 13, a modem 5B, a communication control 14, and a call request switch 21. The voice communication control code detecting portion 15 makes the L relay 16 operate when the detecting portion 15 detects a receiving code. As a result, an l contact 17 operates so that the line 3 is connected to the speaker 20. In such a way, the operator in the center 1 can send his voice to the passenger in the elevator through the speaker 20.
In case where, in the center, voice communication state is switched to a receiving state, the operator in the center 1 returns the press-to-talk switch 10 to the former state, so that the monostable multivibrator 11 is again triggered at the trailing edge and makes the M relay 7 perform a timing operation just as the above described case and also makes the input B to the communication control 6 become a logical "H". Since the input A is a logical "H" (a press-to-talk switch 10 is off) and the input B is a logical "H", the communication control 6 sends out to the emergency call unit 2 a transmission code which is a voice communication control code for switching the unit 2 to a transmission side. The emergency call unit 2 is controlled to be switched to a transmission side in the above described manner.
Since a conventional data communication system is structured as described in the foregoing, a voice signal from the microphone 19 is applied to the modem 5B through the hybrid coil 13 at the time of transmission from the emergency call unit 2 to the center 1, which causes an S/N ratio of an input of the modem 5B to be decreased. Therefore, if an incoming level from the line 3 is in a lower state, for example, less than -30dBm, the modem 5B frequently outputs an error code making an erroneous control or a non-control, which is a significant defect.